The biological treatment of waste water can essentially take place in two ways, i.e. aerobically by making use of microorganisms which use oxygen, and anaerobically by growth of microorganisms in the absence of oxygen. Both methods have found their place in the art of waste water treatment. The first method is used mainly when there is a low degree of contamination, at low water temperature and as a polishing treatment. The second method offers advantages especially as a pretreatment for more severe organic contamination and at higher water temperature. Both methods are adequately known.
Nowadays anaerobic reactors are frequently placed in series with aerobic reactors. such as, for example:
1. compact anaerobic pretreatment followed by aerobic after-treatment ("polishing") for extensive removal of BOD/COD; PA0 2. nitrification followed by denitrification for extensive removal of nitrogen; PA0 3. sulphate reduction followed by oxidation of sulphide to elemental sulphur for removal of sulphur.
To an increasing extent the existence of aerobic and anaerobic reactions which take place simultaneously in the same reactor is also being reported. Examples of these are nitrification/idenitrification reactions and denitrification under the influence of sulphide oxidation.
Anaerobic processes can also be the reason for low sludge growth figures in highly loaded aerobic systems. The use of a relatively low oxygen pressure in a reactor containing agglomerated (flocculated) biomass can lead to rapid conversion of oxygen-binding substances by aerobic bacteria which are present in the outer layer of the flocs. These bacteria preferably store the nutrition as reserves in the form of polysaccharides outside their cells.
Subsequently the bacteria get no opportunity to use these reserves because of lack of oxygen, and these reserves therefore start to serve as a substrate for anaerobic mineralization processes in the interior of the floc, where no oxygen can penetrate. As a result a simultaneous build-up and break-down of the polysaccharides is produced, the polysaccharides also serving as an adhesive for the cohesive bacterial culture. Protozoa can also play an important role as bacteria-consuming predators with a low net sludge yield.
In this context the term micro-aerophilic is indeed used to indicate that less oxygen is fed to the system than would be necessary by reason of a complete aerobic reaction. This has the result that a bacterial population develops which can multiply under a very low oxygen pressure. A disadvantage of these conditions can be that foul-smelling substances can be produced, such as H.sub.2 S, NH.sub.3 or volatile organic acids. These can be stripped off by air bubbles and pass into the outside air. It can therefore be important that this air is collected for treatment if necessary.
On the other hand, it is important that sufficient inoculating material remains present in the reactor and that the flocs formed are not flushed out before the anaerobic mineralization processes have taken place.
Recent research has revealed that anaerobic bacteria can have a high tolerance to oxygen (M. T. Kato, Biotech. Bioeng. 42: 1360-1366 (1993)). The addition of oxygen can sometimes also be advantageous for an anaerobic process, for example for suppressing sulphate reduction in fermentation tanks, as described in EP-A 143 149. in this latter process, organic solid waste present in a slurry is converted with the generation of a gas which contains methane as the main constituent, also containing a small proportion of up to 3%, and more particularly 0.1-1.5% by vol., of oxygen.
The retention of biomass in a reactor for waste water treatment is of essential importance for the capacity of said reactor. In conventional aerobic treatment this is usually achieved by continuously returning the sludge (=biomass) separated off outside the reactor, by means of settling, to the aeration tank where the biological reactions take place. This process in which the sludge concentration in the aeration tank is 3-6 g/l, is termed the activated sludge process. The same principle is also applied for the earlier anaerobic treatment systems, although the sludge is then usually separated off with the aid of lamellae separators before it is recycled to the anaerobic reactor chamber. This process is known as the contact process.
An improvement in the anaerobic contact process relates to the use of systems with which sludge retention is achieved in a different way, for example by integrating the settling chamber with the reaction chamber or by counteracting the flushing out of biomass by immobilization on carrier material. It is important for accumulation that the residence time of the sludge is considerable longer than the division time of the various microorganisms. This is particularly important for the anaerobic process, because the growth rate is very low. The development of the "Upflow Anaerobic Sludge Blanket" reactor, known all over the world as the UASB reactor. in the 1970s was an important step forward for anaerobic treatment. The majority of anaerobic treatments are now carried out in this type of reactor.
The characteristic of the UASB reactor is that the effluent to be treated is fed in and distributed over the bottom of a tank, from where it flows slowly upwards through a layer of biomass. During the contact with the biomass, a gas mixture is produced which consists mainly of CH.sub.4, CO.sub.2 and H.sub.2 S; this mixture is known as biogas. The biogas bubbles upwards and thus provides for a certain degree of mixing. As a result of clever positioning of gas collection hoods below the water surface. the gas bubbles do not reach the water surface, with the result that a calm zone is produced at the top and any sludge particles swirled up are able to settle into the layer of biomass (the "sludge blanket") again. The sludge concentration in a UASB reactor is generally between 40 and 120 g/l, usually at 80 to 90 g/l. The UASB reactor is described in many patents, inter alia in EP-A 193 999 and EP-A 244 029. One reason why the UASB reactor has become the most popular anaerobic system is the fact that, with proper process control, the biomass can be allowed to grow in the form of spherical particles a few mm in size which settle very well.
In the meantime extensions or variants based on the principle of the UASB have been proposed which have higher flow speeds, for example as a result of recycling effluent, by using the biogas as an integral pump, or simply by building narrower high columns. The basic principle, however, remains the same as that of the UASB.